1) Field of the Invention
The present invention relates to a gas sensor made to measure an oxygen concentration or the like in an exhaust gas from an engine for a vehicle for the purpose of utilizing the measurement result for the combustion control or the like.
2) Description of the Related Art
The combustion control based on the oxygen concentration or the like in the exhaust gas from a vehicle engine works effectively for the fuel consumption and the efficient exhaust gas purification.
A gas sensor for detecting the oxygen concentration or the like in the exhaust gas internally includes a sensing element for measuring a specific gas concentration of a measured gas (gas to be measured).
This gas sensing element has an electrochemical cell comprising a solid electrolyte and a pair of electrodes and is made to measure an oxygen concentration or the like in a measured gas on the basis of an electromotive force occurring between the electrodes, a limiting current flowing therebetween, or the like with the atmospheric gas being used as a reference gas.
Accordingly, there is a need for the gas sensor to come into contact with both the atmosphere and measured gas, and an outer surface of the gas sensor includes a measured gas surface exposed to the measured gas and an atmosphere surface exposed to the atmosphere. The measured gas surface has an introduction hole for introducing the measured gas into the interior of the gas sensor while the atmosphere surface has an introduction hole for introducing the atmosphere into the interior of the gas sensor.
For example, in the case of a gas sensor shown in FIG. 1 and described later, the gas sensor is inserted into a mounting hole made in an exhaust pipe in which an exhaust gas flows, and a housing is divided halfway so that one division forms an atmosphere surface and the other forms a measured gas surface.
Meanwhile, of the components of a gas sensor, there are materials inferior in thermal resistance. For example, a resin-made water repelling filter (see FIG. 1 and others) is sometimes provided for making the introduction of the atmosphere into the interior of an atmospheric side cover possible but the intrusion of water thereinto preventable. This water repelling filter is made of a porous resin material such as tetrafluoroethylene and its thermal resistance falls below that of a metal or ceramic material.
In addition, an elastic insulating material (see FIG. 1 and others) is sometimes provided for hermetically sealing a base end portion of an atmospheric side cover and for fixing lead wires drawn from the exterior of the gas sensor. However, this elastic insulating material is made of a resin or a rubber and is inferior in thermal resistance.
Furthermore, in recent years, the regulation on the exhaust gas becomes stricter every year and, with this situation, the temperature of the exhaust gas further increases and an outer surface of an exhaust pipe heated by the hot exhaust gas glows, thereby further generating radiant heat.
Since the atmosphere surface of the outer surface of the gas sensor is exposed to the exterior of the exhaust pipe, it can become a radiant heat receiving surface. The radiant heat promotes heating the gas sensor so that there is a possibility of exceeding the thermal resistance limit of a material sensitive to (weak in) heat.
There have been proposed a construction in which an outer surface of the base end of the atmospheric side cover is formed into irregular configuration to form a heat radiating portion or the size of a gas sensor is increased to lengthen the distance between the exhaust pipe and the outer surface for preventing the temperature of the gas sensor from increasing excessively.
However, it is preferable that the gas sensor to be placed in a limited space is made as smaller as possible, and the size enlargement is not desirable also from the viewpoint of the material cost. Moreover, difficulty is experienced in manufacturing the outer surface having the irregular configuration, which leads to lowering the productivity.